1. Field of the Invention
The present invention relates to film scanning systems, and in particular, to frame converters for converting film frame images to video frame image signals.
2. Description of the Related Art
Conventional motion picture film consists of frame images which are commonly displayed sequentially at a rate of 24 frames per second ("fps"), 30 fps or some other rate, as desired. To convert motion picture film images into video image signals, film scanning systems, such as "telecines," have been developed. These systems generate video image signals from motion picture film images by scanning the film frames sequentially to generate film frame image signals. These film frame image signals are then converted into video frame image signals having a standard video frame rate. The standard video frame rate is 25 video frames per second for PAL video, 29.97 video frames per second for NTSC or 30 video frames per second for SMPTE-240M high definition video.
Conventional telecines operate in "real time" in the sense that they generate video frame image signals from motion picture film frame images at the desired standard video frame rate (such as the standard PAL or NTSC video frame rate). The frame rate mismatch between film (24 fps) and PAL video (25 fps) presents virtually no problem. A conventional PAL telecine often scans the motion picture film at the elevated rate of 25 film frames per second for the sake of processing convenience, with little or no problem for the viewers.
However, the frame rate mismatch between the conventional film rate (24 fps) and the NTSC (29.97 fps) or SMPTE-240M high definition video frame rate (30 fps) is significant. Therefore, a technique commonly referred to as "3-2 pulldown" (or "3-2-3-2 pulldown") is often used to generate these higher video frame rates using a telecine. As discussed more fully below, film-to-video frame image conversion using the 3-2 pulldown technique produces actual, or genuine, video frame images (containing original video field images) and simulated video frame images (containing video field images which are duplicates of other original video field images). In other words, simulated video frame images are generated which use duplicate video field images which are, in turn, based upon original film field images. These simulated video frame images are combined with genuine video frame images, thereby producing a video signal which can be displayed, recorded or otherwise processed as desired.
However, if it is desired to convert this video signal back to film, the video-film frame rate mismatch makes it all but impossible. The duplicate video field images within the simulated video frame images produced via the 3-2 pulldown are literally interlaced with the original video field images within the genuine video frame images (which directly represent genuine film frame images), due to the interlaced scanning characteristic of video. Therefore, these simulated video frame images cannot simply be deleted.
Thus, it would be desirable to have a means for identifying the simulated video frame images and duplicate video field images so as to facilitate their removal when converting the video images back to film images.